1. Field of the Invention
The present invention relates to a warning system of a motor vehicle, and particularly to a so-called `oil degradation warning system` which issues a visual and/or audible warning when the oil (viz., engine oil, transmission oil or the like) has been degraded to a certain unusable level. More specifically, the present invention is concerned with the oil degradation warning system which is designed to postpone the issue of warning when replenishment of oil is carried out.
2. Description of the Prior Art
As is known, lubrication oil, such as engine oil, transmission oil or the like, tends to degrade gradually with passage of time for which the oil is practically used. Thus, it is necessary to change the oil when it is degraded to a certain unusable level.
In view of the above, several kinds of oil degradation warning systems have been hitherto proposed and put into practical use. One of them is the engine oil degradation warning system disclosed in Japanese Patent First Provisional Publication No. 59-43299, in which `moved distance` and `operation time` of the vehicle, `oil temperature` and `engine speed` are used as parameters for estimating the degradation degree of the engine oil. That is, these data are momentarily detected, the detected data are added and integrated with passage of time, and when the integrated result reaches to a predetermined limit, a warning for need of oil change is issued.
Apart from the above, it is also known that the amount of the engine oil is gradually reduced because of burning thereof during engine operation. Thus, oil replenishment is necessary under practical use of the vehicle. With the replenishment, the concentration of impurities in the oil is reduced thereby substantially lowering the oil degradation degree, that is, reviving the oil. However, since, in the afore-described warning system, the warning time (viz., the time at which the warning is issued) is determined by only considering the automatically detected informations or data (viz., moved distance, operation time, oil temperature and engine speed) on vehicle operation history, the oil replenishment permits the warning device to issue the warning considerably before the time when the oil change is really needed. That is, in an extreme case, it occurs that the oil change warning is issued on the day subsequent to the oil replenishment.